DishTV set top box -- some doubts.

I have a three year old DishTV connection. The Set-top-box(standard definition) has become very unreliable and I want to buy a new one. I have recently recharged for almost 15 months. Is it possible to buy a new set-top-box and continue to use old connection?(I don't to loose 15 months validity). Also, if I buy a HD STB, will the existing card work with it?

Thanks in advance